The Anunnaki come from the planet Nibiru.
They came to earth to mine gold, which they require to power their civilization.
They created and enslaved humanity to do the hard work for them.
All the familiar players from Sumerian mythology—Enlil, Enki, Anu, Marduk and the rest—were actually alien administrators.
Nibiru is in a long orbit around the sun.
The last time it came close to Earth, it upset the tides, causing the great flood.
Eventually Nibiru will return, causing another cataclysm.